The present invention relates to a compact zoom lens for use in digital still cameras, video cameras, and other devices that are equipped with an imaging element having a high pixel count such as a CCD.
Japanese Laid-Open Patent Publication Number Hei 5-173073 and Japanese Laid-Open Patent Publication Number Hei 6-201993 disclose conventional zoom lenses that include a first lens group with a negative refractive power; a second lens group with a positive refractive power; and a third lens group with a positive refractive power. The three lens groups include at least seven lenses arranged along an optical axis.
In recent years, there have been significant developments in the technology of solid-state imaging elements such as CCD""s that are used in devices such as digital still cameras and video cameras. The increase in the density and number of pixels in these devices has led to a growing need for lenses for optical systems with superior optical characteristics. However, as devices such as digital still cameras and video cameras decrease in size, the zoom lenses mounted on these devices must become more compact, thinner, and lighter. Compactness, especially in terms of the length of the lens, is particularly important when the zoom lens is mounted on the main camera unit regardless of whether the camera is in use.
A conventional zoom lens uses at least seven lenses which requires that the zoom lens has a significant length along its optical axis. As a result, conventional zoom lenses do not meet the demands of more compact, thinner digital still cameras, video cameras, and the like.
The present invention provides a zoom lens having a compact, light, and thin design with superior optical characteristics that efficiently correct various aberrations. The zoom lens is used particularly for imaging elements with high pixel counts, wherein the zoom factor is approximately 3 and the following conditions are met: the total lens length during shooting is no more than 37 mm; the total thickness along the optical axis of the lens groups is no more than 15 mm; the back focus for placement of a low-pass filter and the like is at least 3 mm; the lens brightness (F number) at the wide-angle end is approximately 2.8; and the distortion is no more than 5%.
The zoom lens includes, from the object side to the image plane side: a first lens group having a negative overall refractive index, a second lens group having a positive overall refractive index, and a third lens group having a positive overall refractive index. The first lens group is formed from a first lens having a negative refractive power and a second lens having a positive refractive power. The second lens group is formed from a third lens, a fourth lens and a fifth lens. The third lens has a positive refractive power. The fourth and fifth lenses are joined together and have a negative refractive power. The third lens group is formed from a sixth lens having a positive refractive power.
With this structure, the total system can be formed with six lenses. Therefore, the total length and the retracted size of the zoom lens are reduced, and it is compact and thin.
The zoom lens as described above can satisfy the following conditions (1) and (2):
0.5 less than f2/|f1| less than 1.2xe2x80x83xe2x80x83(1)
1.5 less than f3/fw less than 6xe2x80x83xe2x80x83(2)
where fi (i=1xe2x88x923) is the focal distance of the i-th lens group and fw is the focal distance of the total lens system at the wide-angle end. Distortion, lateral chromatic aberration, and the like are corrected, and the zoom lens is telecentric and compact. Other optical characteristics are also improved with this construction.
The zoom lens as described above can also satisfy the following conditions (3) and (4):
v1xe2x88x92v2 greater than 10xe2x80x83xe2x80x83(3)
D2/fw greater than 0.2xe2x80x83xe2x80x83(4)
where v1 is the Abbe number of the first lens, v2 is the Abbe number of the second lens, D2 is the distance between the first lens and the second lens along the optical axis, and fw is the focal distance of the total lens system at the wide-angle end. Spherical aberration, astigmatism, distortion, and the like are corrected when these conditions are satisfied. This configuration is particularly effective for correcting chromatic aberration.
The first, third, and sixth lenses can include aspherical surfaces to correct aberrations such as spherical aberration, coma, and the like.
The aspherical surface of the first lens can be formed at the end having a smaller curvature radius or can be formed with a diminishing refractive index toward the periphery. These modifications can correct distortion.
The first lens group, the second lens group, and the third lens group can move along the optical axis during the zooming and image plane correction operations performed from the wide-angle end to the telescopic end. This structure allows for the correction of different types of aberrations at the central focal distance at a central position between the wide-angle end and the telescopic end.
The third lens group can move along the optical axis toward the image plane side and then toward the object side during the zooming and image plane correction operations performed from the wide-angle end to the telescopic end. This structure allows for the correction of astigmatism at the central focal distance at a central position between the wide-angle end and the telescopic end.
The features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings.